U.S. provisional application 60/214,560
U.S. provisional application 60/214,562
This invention is directed to heating/defrost cycles of a very low temperature refrigeration system, and more particularly, to an improved heating cycle incorporating a defrost supply loop and a defrost return bypass loop for optimizing the heating/defrost cycle, for preventing overload (excessive pressure) of its refrigeration process and thereby allowing the defrost cycle to operate continuously, for shorter recovery period between heating/defrost and cooling operating modes, for controlled flow where the rate of the temperature change during cool down or warm up is controlled in an open loop fashion.
Refrigeration systems have been in existence since the early 1900s, when reliable sealed refrigeration systems were developed. Since that time, improvements in refrigeration technology have proven their utility in both residential and industrial settings. In particular, low-temperature refrigeration systems currently provide essential industrial functions in biomedical applications, cryoelectronics, coating operations, and semiconductor manufacturing applications. In many of these applications, it is necessary that refrigeration systems not only need to provide low temperatures but also undergo a defrost cycle in which the system is brought to a temperature well above 0xc2x0 C. The company that develops the refrigeration systems that can perform across this range of temperatures and own the related intellectual property stands to reap substantial gains.
Providing refrigeration at temperatures below xe2x88x9250 C has many important applications, especially in industrial manufacturing and test applications. This invention relates to refrigeration systems which provide refrigeration at temperatures between xe2x88x9250 C and xe2x88x92250 C. The temperatures encompassed in this range are variously referred to as low, ultra low and cryogenic. For purposes of this Patent the term xe2x80x9cvery lowxe2x80x9d or very low temperature will be used to mean the temperature range of xe2x88x9250 C to xe2x88x92250 C.
In many manufacturing processes conducted under vacuum conditions, and for a variety of reasons, the heating of a system element is required. This heating process is known as a defrost cycle. The heating elevates the temperature of the manufacturing system, enabling parts of the system to be accessed and vented to atmosphere without causing condensation of moisture in the air. The longer the overall defrost cycle and subsequent resumption of producing very low temperatures, the lower the throughput of the manufacturing system. Enabling a quick defrost and a quick resumption of the cooling of the cryosurface in the vacuum chamber is beneficial. What is needed is a way to increase the throughput of a vacuum process.
There are many vaccuum processes which have the need for such very low temperature cooling. The chief use is to provide water vapor cryopumping for vacuum systems. The very low temperature surface captures and holds water vapor molecules at a much higher rate than they are released. The net effect is to quickly and significantly lower the chamber""s water vapor partial pressure. Another application involves thermal radiation shielding. In this application large panels are cooled to very low temperatures. These cooled panels intercept radiant heat from vacuum chamber surfaces and heaters. This can reduce the heat load on surfaces being cooled to lower temperatures than the panels. Yet another application is the removal of heat from objects being manufactured. In some cases the object is an aluminum disc for a computer hard drive, a silicon wafer for an integrated circuit, or the material for a flat panel display. In these cases the very low temperature provides a means for removing heat from these objects more rapidly than other means, even though the object""s final temperature at the end of the process step may be higher than room temperature. Further, some applications involving, hard disc drive media, silicon wafers, or flat panel display material involve the deposition of material onto these objects. In such cases heat is released from the object as a result of the deposition and this heat must be removed while maintaining the object within prescribed temperatures. Cooling a surface like a platen is the typical means of removing heat from such objects. In all these cases it is to be understood that the evaporator surface is where the refrigerant is removing heat from these customer applications when providing cooling at very low temperatures.
In many refrigeration applications, a high temperature for a longer period is needed to allow for a slow response time of the item being heated. With extended defrost times, conventional systems get overloaded and shut down due to high discharge pressures ranging from 300 to 500 psi. The system""s compressor""s discharge pressure needs to be limited to protectagainst excessive discharge pressures; otherwise, downstream components are over-pressurized. Typically, a safety switch or pressure relief valve is in place to prevent excessive discharge pressure; however, this inhibits the defrost cycle. What is needed is a way to increase the defrost time of a refrigeration system without exceeding its operating limits.
In many applications, gradual heating or cooling may be required. For example, rapid temperature changes in a ceramic chuck of a semiconductor wafer manufacturing process cannot exceed certain limits that vary based on the specific material properties of the chuck. If this rate is exceeded, the chuck will crack. What is needed is a way to provide a variable heating and cooling system.
Conventional very low temperature refrigeration systems have a normal defrost time ranging typically from 2 to 4 minutes, and as much as 7 minutes for a large coil. With these defrost times, the refrigeration system is strained due to the high discharge pressures, therefore requiring a 5-minute recovery period before cooling can be resumed, and extending the overall defrost cycle. What is needed is a way to shorten the overall defrost cycle of a refrigeration system.
A bakeout process is the heating of all surfaces in a vacuum chamber to remove water vapor in the chamber after it has been exposed to the atmosphere (such as when the chamber is opened for maintenance). Conventional techniques of performing a bakeout process involve heating the surfaces with a heater that exposes the vacuum chamber components to above 200xc2x0 C. for a prolonged period of time to expedite outgassing of water vapor from the chamber surfaces. If a cooling surface is in a chamber being heated with this method the remaining refrigerants and oils consequently break down, thus decreasing the reliability of the refrigeration process. What is needed is a way to maintain the chemical stability of the process fluids during a bakeout process.
U.S. Pat. No. 6,112,534, xe2x80x9cRefrigeration and heating cycle system and method,xe2x80x9d assigned to Carrier Corporation (Syracuse, N.Y.), describes an Improved Refrigeration System and Heating/Defrost Cycle. The system, for heating circulating air and defrosting an enclosed area, includes a refrigerant, an evaporator using said refrigerant for heating the circulating air; and a compressor for receiving the refrigerant from the evaporator and compressing the refrigerant to a higher temperature and pressure. The system further includes the combination of an expansion valve positioned between the compressor and the evaporator for forming a partially expanded refrigerant, a controller for sensing system parameters, and a mechanism responsive to said controller, based on the sensed parameters, for increasing temperature differential between the refrigerant and the circulating air, for improving system efficiency and for optimizing system capacity during heating and defrost cycles.
U.S. Pat. No. 6,089,033, xe2x80x9cHigh-speed evaporator defrost system,xe2x80x9d assigned to Dube, Serge (Quebec, Canada), describes a high-speed evaporator defrost system comprised of a defrost conduit circuit connected to the discharge line of one or more compressors and back to the suction header through an auxiliary reservoir capable of storing the entire refrigerant load of the refrigeration system. Auxiliary reservoir is at low pressure and is automatically flushed into the main reservoir when liquid refrigerant accumulates to a predetermined level. The auxiliary reservoir of the defrost circuit creates a pressure differential across the refrigeration coil of the evaporators sufficient to accelerate the hot high pressure refrigerant gas in the discharge line through the refrigeration coil of the evaporator to quickly defrost the refrigeration coil even at low compressor head pressures and wherein the pressure differential across the coil is in the range of from about 30 psi to 200 psi
U.S. Pat. No. 6,076,372, xe2x80x9cVariable load refrigeration system particularly for cryogenic temperatures,xe2x80x9d assigned to Praxair Technology, Inc. (Danbury, Conn.), describes a method for generating refrigeration, especially over a wide temperature range including cryogenic temperatures, wherein a non-toxic, non-flammable and low or non-ozone-depleting mixture is formed from defined components and maintained in variable load form through compression, cooling, expansion, and warming steps in a refrigeration cycle.
U.S. Pat. No. 5,749,243, xe2x80x9cLow-temperature refrigeration system with precise temperature control,xe2x80x9d assigned to Redstone Engineering (Carbondale, Colo.), describes a low-temperature refrigeration system (10) for accurately maintaining an instrument (11) with a time varying heat output at a substantially constant predetermined cryogenic temperature. The refrigeration system (10) controls the temperature of the instrument (11) by accurately adjusting the pressure of coolant at a heat exchanger interface (12) associated with the instrument (11). The pressure and flow of coolant is adjusted through the use of one or two circulation loops and/or a non-mechanical flow regulator (24) including a heater (32). The refrigeration system further provides a thermal capacitor (16) that allows for variation of the cooling output of the system (10) relative to a cooling output provided by a cooling source (14).
U.S. Pat. No. 5,396,777, xe2x80x9cDefrost controller,xe2x80x9d assigned to General Cryogenics Incorporated (Dallas, Tex.), describes a method and apparatus to refrigerate air in a compartment wherein liquid CO2 is delivered through a first primary heat exchanger such that sufficient heat is absorbed to evaporate the liquid carbon dioxide to form pressurized vapor. The pressurized vapor is heated in a gas-fired heater to prevent solidification of the pressurized carbon dioxide when it is depressurized to provide isentropic expansion of the vapor through pneumatically driven fan motors into a secondary heat exchanger. Orifices in inlets to the fan motors and solenoid valves in flow lines to the fan motors keep the vapor pressurized while the heater supplies sufficient heat to prevent solidification when the CO2 vapor expands through the motors. CO2 vapor is routed from the second heat exchanger to chill surfaces in a dehumidifier to condense moisture from a stream of air before it flows to the heat exchangers.
The present invention is a controlled very low temperature refrigeration system with the capability for long term cooling as low as xe2x88x92150 C and long term heating as high as +130 C using a single evaporator. During an extended defrost mode, the very low temperature refrigeration system does not allow the defrost gas to return to its refrigeration process unit continuously. Instead, the very low temperature refrigeration system of the present invention allows a return bypass, preventing overload (excessive pressure) of its refrigeration process, and thereby allowing the defrost cycle to operate continuously. In a cooling mode, however, the defrost return bypass may be utilized while the cooling surface is being cooled down, thereby enabling a shorter recovery period. Because the very low temperature refrigeration system of the present invention permits a shorter recovery period after each defrost cycle, the total processing time can be reduced. Additionally, there is controlled flow in the very low temperature refrigeration system of the present invention where the rate of the temperature change during cool down or warm up is controlled in an open loop (i.e. without controller feedback) fashion. Furthermore, the very low temperature refrigeration system of the present invention takes advantage of the fill temperature spectrum available in the system to provide constant or variable refrigerant supply and/or return temperatures in a controlled fashion.
For a better understanding of the benefits of the controlled very low temperature refrigeration system of the present invention, a brief discussion of a conventional very low temperature refrigeration system is provided below.
Typically, conventional very low temperature refrigeration systems have a defrost function that warms an evaporator surface, such as a coil or stainless steel platen, to room temperature within a few minutes. A short defrost cycle, typically 2 to 4 minutes, adds value to the product because the shorter time required to go from cool to warm allows good use of equipment for the user, i.e., allows higher product throughput.
In a typical defrost cycle, the refrigerant in the evaporator is warmed only to room temperature, which works well with coils but not with other types of surfaces (i.e., stainless steel platen) where there is not a large thermal interface between the evaporator surface (i.e. platen surface) and the refrigerant. Secondly, a stainless steel platen has a long response time. Even though a defrost cycle occurs and the coolant returns from the platen at room temperature or higher, the platen is still cold because of poor response time. As a result, only a portion of the platen has been warmed, and upon completion of the defrost cycle the platen is still colder than acceptable. Consequently, a longer defrost cycle is desirable. However, current designs of refrigeration systems are limited and do not allow extended defrost time because the system becomes overloaded and shuts down due to high discharge pressure. Typically, a safety switch or pressure relief valve on the discharge side is in place to prevent excessive discharge pressure and possible system damage. Therefore, a longer defrost cycle (using the traditional method) is not possible within the confines of the operating limits of conventional very low temperature refrigeration systems.
The present invention provides a means to provide extended operation in defrost and to prevent the system from experiencing excessive discharge pressures. To accomplish this, a method of bypassing the flow of warm returning refrigerant gas around the refrigeration process is used. The goal of this approach is to use standard refrigeration components for this bypass branch. However, such standard components are not rated for exposure to very low temperature fluids. Operation of these components at very low temperatures will result in failure of elastomer seals, loss of mechanical properties important to assuring proper pressure ratings of the valves and compressor housing due to embrittlement of some alloys at low temperature. The invention describes how to use these standard components in such a way that they are not exposed to very low temperatures.
On the other extreme, very high temperatures can also damage components. Specifically the refrigerant and compressor oil which are always present to some extent in the evaporator, when the evaporator is connected to the refrigeration system. During the bakeout of the vacuum chamber the evaporator could be exposed to temperatures of 200 C or higher. This exceeds the maximum exposure temperature of the refrigerant and oil. Prolonged exposure to these temperatures will result in chemical breakdown of these molecules. The resulting products contain acids which will cause shortened life of key system components such as the compressor. Providing a means of circulating hot refrigerant at +130 C or less through the evaporator in the defrost mode assures that the refrigerant and the oil in the evaporator stay within temperature limits to prevent any chemical decomposition.
Still other objects and advantages of the invention will be apparent in the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangements of parts, which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.